[volt-nuts] Fluke 335A versus HP340B

Dick richiem5683 at gmail.com
Sat Dec 15 05:17:53 UTC 2012

Hi David -- I have and like an 887AB -- I took the ruined batteries out, and replaced the mechanical chopper amp with an OPA111 BiFet opamp that was very spendy at the time, but worked really well -- probably an OPA227 would be good these days. The only liability of the 887 and most of the other Fluke Diff meters is that at 100V and 1kV, they do not have very high input impedance, due to voltage dividers which set the input R to 10Mohm. At 10V and 1V, input R is near infinite at null. The Fluke analog meters with near infinite R on all ranges are the 895A (I think one of the best ever) and the 893A (only 5 digits, but very good). Bill is right, the 845AB is the gold standard. I am not familiar with the Keithley.

The 332 and 335 in my experience both suffer from zero instability in the DC output, but they do have the Motorola/Fluke transistor+zener reference in an oven. There is a German website that has a nice replacement circuit for the 332D chopper amp plug-in board that uses an LTC1150 followed by an LT1001 -- this improves both zero point and full range set point stability. The site is Friedrich-MesseTechnik (you may need to register and log in to see the diagram)  --


Dick Moore

On Dec 14, 2012, at 7:13 PM, volt-nuts-request at febo.com wrote:

> Message: 8
> Date: Fri, 14 Dec 2012 19:13:01 -0800
> From: WB6BNQ <wb6bnq at cox.net>
> To: Discussion of precise voltage measurement <volt-nuts at febo.com>
> Subject: Re: [volt-nuts] Fluke 335A versus HP 740B
> Message-ID: <50CBEABD.1C843C8 at cox.net>
> Content-Type: text/plain; charset=us-ascii
> David,
> I had to go research a bit.  The primary difference between the A & D is output accuracy and stability.  Roughly a 2 to 1 improvement.  Keep in mind we are talking splitting hairs.  Either one would exceed what one would
> normally need for a small home lab.
> As for the Fluke 845, you simply cannot beat it.  If it is not outrageous, grab it.  Hopefully, it is the portable version which had the highest of isolation rating and gives you freedom from ground loops because it runs on
> batteries, but the rack mount version is nice as well ( I have both models).
> The Fluke 887 is a nice meter as well.  They were very good in their time.  However, a top of the Fluke or HP DVM would give them a run for their money.  If the price is LOW, it wouldn't hurt to have one.  The big draw back is
> having to operate all the switches to get a result.  If your constantly operating the switches, they will wear out.  So, it is not what I would consider a general purpose meter, but more of a backup reference standard.  For
> normal stuff just use a cheap DVM.
> I have all the above equipment except Differential voltmeter is an earlier version, the 883AB, which is the same as the 887 with slightly less accuracy.
> Bill....WB6BNQ

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